Scientists Seek Source Of Life's Lopsided Left-Handedness

By Christopher Paul

This is an artist’s concept of excess left-hand aspartic acid created in asteroids and delivered to Earth via meteorite impacts. The line at the bottom is a chromatogram showing that left-hand aspartic acid (tall peak in the center, with diagram of left-hand aspartic acid molecule on top) was four times more abundant in the meteorite sample than right-hand aspartic acid (smaller peak to the left, with right-handed aspartic acid molecule on top). Image Credit: NASA/Hrybyk-Keith, Mary P.

A recent analysis of a meteorite recovered in Canada sheds new light on a well-known mystery in biology.

The meteorite fell to Earth in January 2000. It exploded over British Columbia, Canada and pieces fell across the frozen Tagish Lake. Many people witnessed the meteor fall and collected pieces quickly and kept them frozen to minimize contamination by Earth organisms.

A team led by Dr. Daniel Glavin of NASA’s Goddard Astrobiology Analytical Laboratory at the Goddard Space Flight Center in Maryland, has analyzed some of those fragments and made an exciting discovery.

Glavin’s team put small samples of the Tagish Lake meteorite into hot water solutions, drawing out any volatile molecules, then fed them into a mass spectrometer, a device that determines the composition of molecules by ionizing them and determining their charge-to-mass ratio.

The team was not surprised when organic molecules showed up in the analysis, space rocks have long been known to carry pre-biotic organic molecules, but they were surprised to discover that most of the aspartic acid was of the left-handed variety.

Many molecules, including amino acids, which are the building blocks for proteins and are vital for all life on Earth, have a property known as “chirality,” from the Greek word for hand.

This means that there are two versions of each molecule, a “right-handed” version and a “left-handed” version. These two molecules are mirror images of each other. Just like human hands, they are perfect mirror images, but can never be superimposed on each other.

Chiral molecules are divided into right-handed and left-handed classes, based on certain factors about their structure.

Meteorites like this one discovered at Tagish Lake are providing invaluable insights in molecular biology. Photo Credit: Michael Holly, Creative Services, University of Alberta.

Amino acids are a well-known class of chiral molecule.

Normal, non-biologic chemical reactions should produce equal numbers of right- and left-handed versions of chiral molecules, but life on Earth uses left-handed amino acids almost exclusively.

This has long been a mystery for biologists and chemists alike. In particular, if non-biologic chemical reactions produce right- and left-handed versions equally, why do the organisms that were born from those reactions choose to use the left-handed versions?

Glavin and his team think they may have found the answer.

Analysis of the aspartic acid, an amino acid that is vital to human life, revealed that there were four times more left-handed molecules than right-handed ones.

But another amino acid found in the meteorite sample, alanine, showed no such discrepancy. Glavin believes this is evidence that the samples were not contaminated by Earth chemistry, reasoning that if Earth chemistry had contaminated the samples, that both molecules would have a large discrepancy between right- and left-handed molecules.

Glavin also reasons that since these molecules are rich in carbon-13, an isotope of carbon that has one extra neutron, they are likely from space. Carbon-13 is rare on Earth, but is more plentiful in space, where energetic particles may interact with normal carbon-12 to create carbon-13.

Previous theories about the creation of more left-handed than right-handed amino acids in space involved expose of the pre-biotic molecules to polarized radiation in the solar nebula early in the Solar System’s history. But this theory alone cannot account for the massive discrepancy in the number of right- and left-handed aspartic acid molecules.

Glavin proposes another theory; aspartic acid tends to crystallize in either right-handed or left-handed forms. The two versions do not generally mix during the crystallization process.

But alanine prefers to mix the two versions when crystallizing.

Glavin believes this preference for single-version crystals accounts for the majority of left-handed molecules of aspartic acid. He proposes that early on in the Solar System’s history, polarized light in the solar nebula, or some other process, produced a slight excess of left-handed aspartic acid molecules. These molecules crystallized, and the left-handed crystals catalyzed the creation of more left-handed molecules.

Meanwhile, even if there was an excess of left-handed alanine molecules, the acid’s tendency to form mixed crystals evened out the discrepancy.

This process may even have occurred on Earth in places where such molecules could form, a similar process may have driven the creation of an excess of left-handed amino acids. This excess, fueled by both space rocks falling to Earth and processes inside the Earth itself may have caused life to prefer left-handed over right-handed amino acids.

While this theory may make it easier to understand how life got its start on Earth, it makes it more difficult to find conclusive evidence of past or present life beyond the Earth. Since non-biologic chemistry can drive the formation of an excess number of left-handed molecules, the detection of such an excess, which might have lead to a belief that evidence of life had been found, does not necessarily mean that biological chemistry created that excess.

Dr. Glavin is the lead author of this paper, which will be published in the Meteoritics and Planetary Science journal.